In-situ tomography of 3D photopolymerization
Tomographic additive manufacturing physically reverses computed tomography for 3D printing.
The reactive transport of oxygen during photopolymerization controls the quality of the printout and usually there exists a short time window for having the optimum light exposure.
Optical tomography as a non-invasive 3D imaging technique can be employed for in-situ monitoring of the refractive index variations to find the window for termination of the light exposure to achieve the optimum print quality.We have designed a visible light optical tomography system equipped with an in-house image reconstruction toolbox to monitor the density variations during tomographic volumetric printing (TVP).
Developed software follows an automated two-step workflow including image processing and a moving-window based dynamic reconstruction using filtered backprojection (FBP).
The main assumption in FBP is that the light rays always travel in parallel, and absorption is the only dominant mechanism in image formation. However, this assumption results in partial reconstruction of the geometry evolution mainly because of the continuous refractive index mismatch that occurs between the solidified region and its surrounding liquid.
To reconstruct the true geometry, we have designed an experiment in which we dissolve a fluorescent dye inside the liquid monomer. The 3D printed workpiece is then used to perform emission imaging instead of trasmission. The emitted signals from the workpiece are filtered using a band-pass filter and are focused on the camera focal plane using optical lenses.
The reconstructed geometry is compared to the digital 3D model to assess the print quality and paves the way toward developing a feedback loop for further quality improvement.